Photochromic glasses or phototropic glasses, as such have been variously designated, had their genesis in U.S. Pat. No. 3,208,860. Such glasses become darker (change color) when subjected to actinic radiation, customarily ultraviolet radiation, and fade or assume their original color when removed from the actinic radiation. That patent discloses the utility of silver halide crystals, i.e., silver chloride, silver bromide, and/or silver iodide, in causing the phenomenon and postulates an explanation of the mechanism generating photochromic behavior in those glasses containing silver halide crystals. The patent is directed generally to silicate glasses and specifies minimum amounts of silver and halogens which must be present to impart photochromic behavior thereto. Thus, at least one halogen in the indicated minimum effective proportion of 0.2% chlorine, 0.1% bromine, and 0.08% iodine and a minimum of silver in the indicated proportion of 0.2% in a glass wherein the effective halogen is chlorine, 0.05% in a glass containing at least 0.1% bromine, but less than 0.8% iodine, and 0.03% in a glass containing at least 0.08% iodine must be present.
The patent notes the preferred base glass compositions as being included within the R.sub.2 O--Al.sub.2 O.sub.3 --B.sub.2 O.sub.3 --SiO.sub.2 system. Accordingly, the preferred base glass compositions consist essentially, expressed in weight percent on the oxide basis, of about 40-76% SiO.sub.2, 4-26% B.sub.2 O.sub.3, 4-26% Al.sub.2 O.sub.3, and at least one alkali metal oxide (R.sub.2 O) in the indicated proportion of 2-8% Li.sub.2 O, 4-15% Na.sub.2 O, 4-15% K.sub.2 O, 8-25% Rb.sub.2 O, and 10-30% Cs.sub.2 O, the sum of those components plus the silver halides constituting at least 85% by weight of the total composition.
Finally, the patent observes that, where a transparent photochromic glass article is desired, the composition thereof will not contain more than 0.7% silver or more than 0.6% total of the three halides, and the content and size of the silver halide crystals will not exceed 0.1% by volume and 0.1 micron, respectively.
Ophthalmic lenses, both as prescription lenses and as non-prescription sunglasses, have comprised the largest commercial application of photochromic glass. Prescription lenses, marketed by Corning Glass Works, Corning, New York under the trademark PHOTOGRAY, have formed the greatest segment of the ophthalmic sales. The composition of that glass falls within the disclosure of U.S. Pat. No. 3,208,860, supra, an approximate analysis therefor in weight percent being:
______________________________________ SiO.sub.2 55.6 B.sub.2 O.sub.3 16.4 Al.sub.2 O.sub.3 8.9 Li.sub.2 O 2.65 Na.sub.2 O 1.85 K.sub.2 O 0.01 BaO 6.7 CaO 0.2 PbO 5.0 ZrO.sub.2 2.2 Ag 0.16 Cu 0.035 Cl 0.24 Br 0.145 F 0.19 ______________________________________
Because PHOTOGRAY brand glass was developed through compromises with respect to such factors as photochromic behavior, ophthalmic properties, the capability for being chemically strengthened, and melting and forming characteristics, research has been ongoing to produce glasses having improved photochromic properties while still retaining the other qualitites necessary for a practical commercial glass.
U.S. application Ser. No. 14,981, filed Feb. 28, 1979 in the names of G. B. Hares, D. L. Morse, T. P. Seward, III, and D. W. Smith, now U.S. Pat. No. 4,190,451 discloses glasses demonstrating improved photochromic behavior when compared with PHOTOGRAY brand glass, i.e., such glasses darken to a lower transmission when exposed to actinic radiation and fade more rapidly when the actinic radiation is removed. That application describes glasses having compositions consisting essentially, in weight percent on the oxide basis, of:
______________________________________ SiO.sub.2 20-65 B.sub.2 O.sub.3 14-23 Al.sub.2 O.sub.3 5-25 P.sub.2 O.sub.5 0-25 Li.sub.2 O 0-2.5 Na.sub.2 O 0-9 K.sub.2 O 0-17 Cs.sub.2 O 0-6 Li.sub.2 O + Na.sub.2 O + K.sub.2 O + Cs.sub.2 O 8-20 CuO 0.004-0.02 Ag 0.15-0.3 Cl 0.1-0.25 Br 0.1-0.2 ______________________________________
wherein the molar ratio alkali metal oxide:B.sub.2 O.sub.3 ranges between about 0.55-0.85 in those compositions which are essentially free from divalent metal oxides other than CuO, and the weight ratio Ag:(Cl+Br) ranges between about 0.65-0.95.
Recently, a new photochromic glass for ophthalmic applications having a composition encompassed within that disclosure was entered into the marketplace by Corning Glass Works, Corning, N.Y. under the trademark PHOTOGRAY EXTRA and cataloged as Code 8111 glass. That glass has the following approximate analysis, in weight percent, of:
______________________________________ SiO.sub.2 55.8 Al.sub.2 O.sub.3 6.48 B.sub.2 O.sub.3 18.0 Li.sub.2 O 1.88 Na.sub.2 O 4.04 K.sub.2 O 5.76 ZrO.sub.2 4.89 TiO.sub.2 2.17 CuO 0.011 Ag 0.24 Cl 0.20 Br 0.13 ______________________________________
In order to better control growth of the silver halide crystals with consequent better uniformity and reproducibility of photochromic behavior, the proper glass-forming batches are melted, the melt cooled sufficiently rapidly to yield a glass shape, and this glass shape thereafter subjected to a heat treatment to nucleate and grow the silver halide crystals. The PHOTOGRAY and PHOTOGRAY EXTRA brand glasses darken to a neutral or gray color when exposed to actinic radiation. However, in light of interest in the marketplace for a photochromic ophthalmic lens which would darken to a brown hue, lens blanks were produced and sold by Corning Glass Works under the trademark PHOTOBROWN. Those lens blanks were prepared via a particular heat treatment being applied to glass compositions within narrowly-defined ranges. A description of their preparation is set forth in U.S. Pat. No. 4,043,781. Thus, glasses consisting essentially, in weight percent on the oxide basis, of
______________________________________ SiO.sub.2 53-60 Al.sub.2 O.sub.3 8-10 B.sub.2 O.sub.3 15-18 Na.sub.2 O 1-3 Li.sub.2 O 1.5-3.2 BaO 5-9 PbO 3.5-7 ZrO.sub.2 0-4 CuO 0.012-0.040 Ag 0.14-0.22 Cl 0.22-0.36 Br 0.10-0.20 F 0-1 ______________________________________
are subjected first to a temperature within the range of 520.degree.-580.degree. C. for about 2-30 minutes and thereafter exposed to a temperature within the range of 600.degree.-660.degree. C. for about 5-60 minutes.
As can readily be observed, the utility of the method is limited to a narrow range of glass compositions and, hence, the breadth of photochromic behavior which can be enjoyed is likewise narrowly limited. For example, the composition of PHOTOGRAY EXTRA brand glass falls outside the purview of that disclosure and, indeed, the patented heat treatment disclosed in U.S. Pat. No. 4,043,781 does not lead to the development of a brownish tint in that glass in the darkened state. Modifications in the disclosed heat treatments can induce a slight brownish cast in that glass in the darkened state but only at the expense of obtaining less desirable photochromic properties.
Of course, the addition of colorants to glass compositions to produce various colorations therein is well known to the art. Hence, for example, a combination of cobalt, nickel, and manganese has been utilized to achieve a brown coloration in glass (U.S. Pat. No. 4,116,704). Also, U.S. Pat. No. 4,018,965 discloses the addition of colorants such as the rare earth metal oxides Er.sub.2 O.sub.3, Pr.sub.2 O.sub.3, Ho.sub.2 O.sub.3, and Nd.sub.2 O.sub.3 and/or the transition metal oxides CoO, NiO, and Cr.sub.2 O.sub.3 to develop various tints in photochromic glass compositions. Thus, that patent discloses the use of up to 1% total of those transition metal oxides selected in the indicated proportions of 0-0.5% CoO, 0-1% NiO, and 0-1% Cr.sub.2 O.sub.3, and/or up to 5% total of those rare earth metal oxides. However, it is apparent that such colorants yield a glass having a permanent tint therein rather than an essentially uncolored glass in the faded state.